1. Field of the Invention
The present invention relates generally to a technique for downloading identical information to plural mobile terminals, and particularly to an information downloading apparatus and a mobile terminal.
2. Description of the Related Art
In the above-described technical field, broadcasting technology for realizing simultaneous transmissive communication with respect to all mobile terminals, or multicast technology for realizing simultaneous transmissive communication with respect to mobile terminals that belong to a predetermined group may be used, for example.
FIG. 1 is a conceptual diagram illustrating an example of information downloading using multicast technology. In FIG. 1, a control channel 10 that is made up of time slots that are respectively assigned to various multicast groups, and a message channel 12 for actually downloading information in relation to the control channel 10 are shown. The control channel 10 is arranged into a repetition of super frames 14, and N time slots that are numbered from 0 to N−1, respectively, are included in each super frame 14. N (>0) may be set according to the number of different types of downloading information that are provided, and designates the number of groups to which a mobile terminal may belong.
FIG. 2 is a table illustrating the relation between a time slot number and a group (information content). For example, if information downloading pertaining to weather forecast is desired, a mobile terminal may be required to subscribe to the multicast group corresponding to slot number 1.
Referring back to FIG. 1, a mobile terminal may receive control channel information pertaining to the group to which it belongs. Such information may only be transmitted through a predetermined timeslot. Thereby, the mobile terminal may conserve its battery energy through. intermittent reception to receive control data from the control channel only during the period corresponding to the time slot of the pertinent group to which the mobile terminal belongs. For example, if the mobile terminal belongs to the weather forecast group, it may receive signals from the control channel during the period of time slot 1, and may be configured to refrain from receiving signals during the periods of the other time slots. The control channel 10 includes information indicating whether download information exists, and the mobile terminal may be configured to extract this information. If it is determined that there is no transmission information, the mobile terminal may go back to a standby mode to receive the next control data from the control channel. On the other hand, if it is determined that transmission information exists, the mobile terminal may receive the transmission information that is transmitted through the message channel 12 after a predetermined time (offset time) elapses from the time slot 1 (i.e., after time t1). Similarly, mobile terminals belonging to other groups may also receive desired information using the control channel that indicates the existence of transmission information and the message channel 12 (e.g., if the desired information corresponds to baseball information, the information is transmitted after time t2; and if the desired information corresponds to stock information, the information is transmitted after time t3).
Japanese Laid-Open Patent Publication No. 2000-224648 relating to information downloading discloses a technique for creating a broadcast channel that is dedicated to broadcasting using a predetermined time slot from time slots provided for a control channel and a communication (message) channel, and transmitting broadcast data via this broadcast channel to thereby simultaneously transmit the broadcast data to plural mobile terminals.
However, in such prior art example, the time slots used for the information downloading implements a fixed signal format that is set beforehand. Thereby, the amount of information to be downloaded and the transmission time are likely to be restricted by the pre-set time slots. To transmit additional information that cannot be accommodated within a predetermined time slot, the information may be divided into different time slots of super frames and transmitted using each time slot assigned to the current group that appears in each super frame 14. The information may then be recombined at the reception side. However, in such case, additional processes are required for dividing and recombining the information. This is inconvenient for a mobile terminal, which is preferably configured to conserve energy consumption. Also, another problem is the fact that much time is required for all the information to be accumulated at the reception side.
Alternatively, instead of dividing and transmitting information in accordance with the format of the super frames, for example, weather forecast information that is originally intended to be transmitted using one time slot may be transmitted using two time slots (i.e., during the period from time t1 to time t3) so that the weather forecast information may be transmitted at once instead of having to be divided. However, in such case, the baseball information may not be transmitted at transmission time t2 as is originally intended (since the weather forecast information is being transmitted), and thereby, this information may not be transmitted until the next transmission time t2 of time slot 2 in the next super frame 14. Accordingly, when attempting to transmit more than the predetermined amount of information, the information downloading time for the current group or another group may be increased, and thereby, the efficient downloading may not always be realized.
It is also noted that information to be transmitted does not always require one time slot's worth of communication resources. However, even in the case of downloading information that does not reach the capacity of one time slot, since a pre-set fixed signal format is used, information resources are allotted in time slot units. Thus, efficient downloading may not always be obtained in regard to communication resource usage efficiency, for example.